Method of making helical thread resistors



Feb. 25, 1947. J, 1 RECTOR I 2,416,347

.METHOD OF MAKING HELICAL THREAD RESISTORS Filed Jan. 4, 1945 INVENTOR.

JACOB L. RECTOR u f l ATTORN EY ases.

Plantea rfb. 25,1941

METHCD QF MAKING HELICAL THREAD v BESISTOBS Jacob' L. Rector, Belmar, N.v J., assignor to the United States of America, as represented by the Secretary of .War

Application January 4, 1945, Serial No. 571,345

Claims. (Cl. 201-67) (Granted under the vact of March 3, 1883,` as amended April 30. 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to resistors and other electrical resistance devices of the type' having terminal ends and means providing a resistive path interposed therebetween. More particularly, the present invention relates, to devices of the character referred to which are adapted for use in high voltage circuits and which must be of uniform precision resistance value.

Heretofore, considerable dimculty has been experienced in manufacturing precision resistors of relatively high resistance value. For example, such resistors have heretofore been made by first coating a smooth cylinder of nonconductive material with a resistive material, and then removing a continuous helical strip of the resistive material, leaving a cylinder threaded by the remaining resistive material. Dimculty has been experienced in achieving precision by this method; and due to the small amount of conductive material used, it has further been found diiilcult to provide uniform lin'ear resistance. Also, resistors so made have been vulnerable to physical injury. It is, therefore, an object of this invention to provide an improved high resistance device of the character referred to in which uniform precision is attained in construction and resistance value while obviating the above and other constructional dimculties heretofore experienced in the manufacturing of such devices. It is also an object of the present invention to provide an improved method of making av high value resistor whose value may be more readily and accurately determined before manufacturing than has heretofore Vbeen possible.

It is a further object of this invention to provide a method of making a high value resistor whose electrical value may be accurately fixed during the manufacturing process.

It is also a further object of provide a method of making a high value resistor that shall have a uniform resistance per unit length.

It is a further object of this invention to provide a resistor device wherein the resistance element thereof is protected from physical injury,

. and 'sufficient dielectric is included between adjacent elements to prevent' arcingat high voltvIt is also an object of thisinvention to provide a resistor that may quickly and easily be aged and otherwise processed for use.

In accordance with the foregoing objects, I have devised a method of making a high value.

this invention to ,f 2 resistor comprising the steps of providing a body having a surface of nonconductive material with a continuous helical groove thread, coating the grooved surface with a resistive material, and then cutting through the surface coating and below the surface of the nonconductive material 'a distance that is greater than the thickness of the coating and less than the depth of said groove whereby a thread oi resistive materialtoform a continuous resistor element is spaced between adjacent convolutions of nonconductive material.

For a further understanding of the invention, reference is made to the following description and accompanying drawings for consideration there* with.

In the drawing, v

Figure 1 is a longitudinal sectional view, partly in elevation, of one embodiment of my invention, representing a preliminary step of manufacture; and

Figure 2 is a view, partially in crosssection, of a resistor embodying the invention and completely manufactured and ready for use.

Referring to Figure 1, core element comprises a threaded cylinder of nonconductive material. A coating 6 is o f any suitable resistive material, such as graphite carried in a binder, which coating completely covers the surface of the core 5. A finish or outer covering 1 of any suitable waterproofing and insulating compound, such as varnish may be applied by spraying or painting over the resistive coating 6. This nish 1 is only applied to the central portion of the cylinder, as will hereinafter appear.

As stated above, Figure 1 represents a preliminary step in the manufacture of this embodiment of my invention. After the external thread is cut into the surface of the dielectric cylinder, the coating of resistive material is applied. This coating may be applied by dipping the cylinder into the resistive material, or the coating may be sprayed or painted onto the surface. The nish is then applied and the product at this point is heat-treated.

'I'he heat treatment serves two purposes. First, it bakes the resistive'coating into a firm unit and hardens the finish, and second, it supplies a series of heat cycles for the purpose of aging the resistor.

Referring now to Figure 2 along with Figure 1, the next step in the manufacture of this resistor consists of the operation of turning down the central portion of the cylinder by any suitable means such as a lathe. A radial distance somewhat less than the depth of the thread is taken for the complete cut, thereby removing the tops of the threads and all resistive material above the depth or the cut and resulting in a central sur face length comprising a hellcally formed strip d of resistive material alternating with and contiguous to a helically formed strip i3 oi fiat topped dielectric. The central portion is then recoated with varnish or other suitable waterprcong and insulating material, as indicated at lil. The coated ends l I which ends have not been covered with the nish l, are not turned down and are pr vided with i'errules 82 and the resistor is ready for use. To insure that the precision desired is obtained, frequent electrical measurements may be made during the turning down process. "lhese measurements, however, are not absolutely necn essary, since the proper depth ci cutting and length of the central portion between the ierrules can be computed beforehand From the foregoing description, it will be seen that a helical resistor element indicated at i8 is disposed in the roots and along the sides oi the truncated threads oi the core il and come prises the -part oi the coating (Figure l) re maining after the central portion oi the body has been machined between the terminal ends. lt will further be seen that this resistor is in the iorm of a helix whose pitch is determined by the pitch of the thread oi the core Finishing coat l@ comprises a coat of varnish or other suitable material as hereinbefore de scribed and is applied alter the turning down process. The whole assembly is now completed and in a form ready for use. Caps il? are stand@ ard terminal ferrules and are pressed on to the ends il of the resistor. lt will be noted that in this embodiment, electrical contact is made leetween the caps l2 and the resistor through that part of coating t (Figure l) that was not removed in the turning down process. A tap id is prof fore any adjustment to the iinal value is madeD thus eliminating all danger from injury by hanl tiling.

While a present preferred embodiment oi the invention has been disclosed and described it is obvious that further modification may he made within the scope oi the invention which is pointed out in the appended claims.

il claim:

l. The method oi melding a resistor compris ing the steps of forming a continuous groove into the surface oi a body oi nonconductive materiel, coating said surface and filling said groove with a layer or resistive material and then uniformly vided for picking od an intermediate value ot v resistance it desired.

The construction described is particularly adapted for use in high voltage, high resistance resistor devices for the reason that the spacing between the threads of resistance material is progressively increased as the depth of the cut in turning down the resistor body, is increased. At the same time the cross sectional area oi the resistance element thus formed is decreased, thereby increasing the resistance of the com= pleted resistor. In any case, the object is to provide such a potential gradient along the resistor from end to end, and such spacing between the turns of the resistor element by the interposed nonconducting body formed by the turned-down tops oi the threads, that arcing-over does not occur between theturns.

The resistivity oi the material applied is such thaty the required resistance um. ru

surface `oi' a cylinder of non-conductive mate` in which 2R is the resistance oi one turn oi diierential thickness, N is the number of turns, 'I' is the thickness factor oi the material, and W is the width of the resistor path.

Once established by trial, the resistance is brought to the proper value by turning the tops of the threads down to a predetermined diameter. The depth to which the cut is made detercutting through said coating and cutting below said surface a distance less than the depth oi the groove..

2. n method of making e. high velue resistor comprising the steps oi providing e. body hav-1 ing n surface oi non=conductive material with a continuous helical groove or thread, and lling the grooved surface with a resistivev material, and then cutting through the surface coating and below the surface or" the nonconductive mate rial e distance that is greater than the thickness of the coating and less than the depth of said groove, whereby a thread ci resistive material to form a continuous resistor element is spaced between adjacent convolutions oi nonconductivc material.

3. n method of ranking a high value resistor comprising steps of providing an elongated body of nonconductive material, grooving the body to provide a thread along the body between the endsthereof, coating the body and piling the grooves with resistive material, protecting a restricted central portion ci the length with. e huish, heat treating the assembly, cutting along said re= stricted central portion through the coating and body to a uniform depth, which point of depth shall be located between the :major and minor diameters oi the thread, to form terminal ends, capping said terminal ends with conductive ele-a ments in. contact with the resistive element, and then applying a second nnishing coat.

t. The method of making a resistor' comprisingl the steps oi cutting a continuous groove into the surface of a body ci non-conductive material, coatingsaid surface and lling said groove with resistive material, heat treating the assembly and then uniformly cutting through said coating and below said surface a. distance less than the depth of the groove. q

.5. The method oi making e. resistor comprising the steps oi cutting a helical thread in the rial, coating said surface and iilling said thread with .a resistive-material, heat treating said assembly wherebysaid resistive material will receive' a permanent set, and then turning down the surface of said cylinder by a radialdistance that is less than the depth of the thread.

.JACOB L. RECTOR.

muumnivcss cum The following references are of record in the file of this patent:

UNITED STATES eATENTs Number Name i Date 1,767,715 Stoeklo June 24, 1930 2,330,782 Morelock Sept. 28, 1943 2,357,473 Jira Sept. 5, `1944 Werner Dec. 29, 1896 

